Background. Our current understanding of the mechanisms for Bcr/Abl-negative myeloprolifereative neoplasia (MPN) involves recognition of restricted MPN driver mutations targeting JAK2, MPL, or CALR, and of an inflammatory microenvironment, characterized by heightened expression and secretion of cytokines and chemokines. However, the contribution of inflammation to MPN progression has not been addressed. Similarly, the causes that lead to an inflammatory microenvironment in MPN have not been defined. Interestingly, the recent observation that clonal hematopoiesis of indeterminate potential (CHIP) exhibit accumulating mutations that correlate with aging, strongly suggests a role for inflammation in clonal selection and disease progression. Preliminary Results. We have generated an animal model of chronic inflammation in which loss of Notch signaling in the BM endothelium (Tie2CreERRBPJKO mice) induces upregulation of the microRNA miR-155, increased NF-kB signaling and pro-inflammatory cytokines resulting in a MPN-like disease within 8-12 months. Importantly, this microenvironmental inflammatory milieu greatly accelerates expansion of hematopoietic cells containing JAK2V617F or TET2-/- mutations. Hypothesis. We propose a model in which: a) prolonged exposure of JAK2V616F or TET2-/- hematopoietic cells to a chronic inflammatory BM niche will favor clonal expansion, acquisition of additional mutations and selection of aggressive clones leading to MPN progression into myelofibrosis or/and AML; b) upregulation of miR-155 plays a pivotal role in generating a vicious cycle of ?malignant? inflammation driving MPN progression. Aims. To test these hypotheses we propose: (1) To determine whether exposure of hematopoietic cells to an inflammatory BM niche leads to clonal selection and disease progression in in vivo models of JAK2V617F and TET2-/- driven MPN; (2) To define the functional relationship between miR-155 and NF-kB in promoting MPN progression, and; (3) To identify an inflammatory signature in the human BM microenvironment correlating with MPN progression. Strategy. To this end, we will use Tie2CreERRBPJKO mice as a genetically-controlled animal model of chronic inflammation, in combination with JAK2V616F or TET2-/- models in transplant settings. In addition, kB-Ras-/- and miR155-/- mice will be used to address the role of miR-155 and NF-kB in disease progression. A lentiviral-DNA barcoding system combined with fluorescent proteins will be used to follow clonal evolution in vivo. This approach will be complemented by whole exome sequencing to identify mutations, and by intravital microscopy, to visualize clones interactions with defined BM niches. Finally, the efficacy of a novel anti-miR-155 therapeutic approach will be tested, and MPN BM specimens will be analyzed to identify inflammatory signatures correlating with disease progression. Relevance. We believe that accomplishment of this work will have impact on the understanding and management of MPN, providing a solid rationale for early therapeutic intervention to suppress chronic inflammation and thus, preventing MPN progression.